Some drunks in a gnu-shaped echo chamber concluded that the world is gnu-shaped. That's not much a joke, if there is one here. Such presently popular axioms as "unix means linux" or "the userland must be gnu" or "bash is installed" can be shown as poor foundations to reason from by using a unix system that violates all those assumptions. That the XCDD comic did not define what a unix system is is another concern; there are various definitions, some of which would exclude both linux and OpenBSD.
edit: maybe i missed the joke?